I. Field of the Invention
The present invention is directed generally to the field of vacuum degassing of liquids and, more particularly, to a method and apparatus associated with removing gases from liquids in a flow-through relation in which an elongated semi-permeable polymeric resin enclosure addresses a vacuum chamber and gas is transferred by diffusion through the enclosure walls. The technique is particularly suited to the removal of air or oxygen from solvent materials associated with liquid chromatograph equipment.
II. Related Art
There are many chemical applications, particularly analytical applications, involving the use of liquid solvents, reactants or the like in which the presence of dissolved gases, particularly air, is undesirable. A prime example of such a situation relates to solvents and chemicals used in liquid chromatography where the presence of even small bubbles of dissolved gases typically interferes with the accuracy and sensitivity of the results obtained. If the dissolved species be chemically active, as is the case with oxygen in air, it can additionally produce unwanted changes or deterioration of the liquid material of interest. Of course, the detrimental effect of the dissolved species is related to the relative concentration of the species in the liquid. These undesirable species usually are removed by a degassing process. It correspondingly follows that the more efficient the removal or degassing system is, the more desirable it will be.
The degassing of liquid materials has been necessary to the success of many processes and, consequently, the process has been pursued actively in several forms for a long period of time. Techniques have included heating or boiling the liquid, exposing the material to a reduced pressure environment or vacuum and using combinations of heat and vacuum to reduce the amount of dissolved gases in the liquid. Ultrasonic energy has also been employed. As conventionally applied, however, these traditional techniques have generally fallen short of the desired degree of separation efficiency. Additionally, a means of degassing solvent involving the passing of a fine stream of bubbles of inert gas such as helium through the solution to be degassed has been shown by Bakalyar et al in U.S. Pat. No. 4,133,767, and in apparatus such as that disclosed by Sims et al in U.S. Pat. No. 4,994,180, co-invented by the inventor in the present application and assigned to the same assignee as the present invention. Helium degassing has been established as the standard for thoroughly removing dissolved gases such as oxygen, nitrogen and other similar components of the atmosphere. Helium sparging for the purpose of degassing, however, has drawbacks such as selectively removing important volatile components of mixed solvents, and requiring large tanks for the supply of helium. Vacuum degassing through a membrane apparatus has been long known, and generally utilizes a membrane permeable only to fixed gases. Degassing by this means is generally established as being capable of reaching 0 concentration of atmospheric gases, and leaves only minor amounts of Helium dissolved in the solvent. For comparison purposes, all tests involved in demonstrating the present invention were run versus Helium degassing as a baseline.
A more recent approach to degassing relatively small volumes of materials involves the provision of the length of relatively small diameter, thin-walled semi-permeable synthetic polymer resin tube contained within an enclosed chamber held under a reduced pressure or vacuum in which the liquid to be degassified is caused to flow through the tube. Amounts of the dissolved gas diffuse through the wall of the tube according to the pressure reduction, permeability, time and area exposed and temperature. These tubes tend to collapse and kink easily, and so must be carefully configured and supported.
Examples of devices of this type include devices such as that illustrated and described in U.S. Pat. No. 5,183,486, in which the tubes exposed within the vacuum chamber conveying the liquid to be degassed are wound in tension on bobbins or spools which support and maintain the inter-coil separation of the rather thin, fragile tubes. Another degassing apparatus is disclosed is U.S. Pat. No. 4,469,495, which employs a coil of semi-permeable polymer resin material containing a plurality of turns. The coil is also quite prone to collapse or kinking and so is required to be supported and the turns separated by a plurality of spacer elements which prevent such physical problems but, unfortunately, in turn, mask a large portion of the area which otherwise could be exposed to the vacuum. Other configurations are shown in U.S. Pat. Nos. 4,430,098, 4,325,715, 3,668,837 and German Offenlegungsschrift 29 07 188.
While each of these devices employs a flow-through tube vacuum degassing approach, and each of the devices has been at least somewhat successful, there remains a need, particularly with devices associated with liquid chromatograph instruments, to make degassing of chemicals and solvents more efficient. One particular limitation or drawback associated with present devices concerns the efficiency of the degassification operation with respect to the tubing itself. Because the tubing must be thin walled to aid diffusion, it tends to collapse and kink readily. In order to overcome these physical limitations, much of the tubing wall surface is masked from the vacuum by apparatus to support and preserve the tubing shape to prevent flow problems. This surface area is lost with respect to the diffusivity. In addition, the efficiency could be further increased if this problem could be overcome and the required wall thickness could be decreased even more.
Accordingly, it is a principal object of the present invention to provide a more efficient liquid vacuum degassing system of the flow-through type using a semi-permeable resin tube that maximizes exposed tubing surface and minimizes required wall thickness.
A further object of the present invention is the provision of a form-memoried, self-supporting helical tube for a flow-through vacuum degassing unit.
A still further object of the present invention is to provide a coiled tube vacuum degassing system which maximizes the exposed tube area by eliminating the need for tube supports and coil turn spacers, thereby minimizing the effect of unusable wall contact areas.
A yet further object of the present invention is to provide a semi-permeable flow-through coil vacuum degassing system that reduces the length of tubing required in the degassing operation.
Another object of the present invention is to reduce the elapsed time to the point the degassified solvent or chemical is available at the output of the device of a semi-permeable flow-through coiled tube vacuum degassing device.
Still another object of the invention is to provide an improved connection system for a synthetic resin semi-permeable tube associated with a flow-through vacuum degassing apparatus.
A still further object of the invention is to reduce the required wall thickness of the tube in a flow-through semi-permeable coiled tube vacuum degassing system.